In one form of the manufacture of high temperature operating hollow air cooled turbomachinery blades, such as airfoil shaped gas turbine engine turbine blades and vanes, a tip cap or end plate is bonded at the open radially outward end to enclose at least a part of the end for control and discharge of a portion of the cooling fluid from within the blade. Such configurations are widely known in the gas turbine engine art, for example as shown in U.S. Pat. No. 3,899,267--Dennis et al. (patented Aug. 12, 1975); U.S. Pat. No. 4,214,355--Zelahy (patented Jul. 29, 1980); and U.S. Pat. No. 4,390,320--Eisworth (patented Jun. 28, 1983). The disclosures of these patents are hereby incorporated herein by reference.
Typical attachment or bonding of such end plates or tip caps to an open blade body end include welding, diffusion bonding, and brazing. The design of certain modern gas turbine engine turbine blades specify that an end plate or tip plate be secured at an inner wall of the open blade end, recessed within the blade tip plenum instead of at the radially outer edge or top of the open blade end. Many current turbine blades are made of relatively highly alloyed Ni base superalloys including relatively large amounts of the elements Al and Ti. Consequently, as manufactured they include relatively large amounts of surface and near or subsurface oxides of such elements which can interfere with the soundness or integrity of subsequently brazed joints.
To achieve good flow of brazing alloy and an acceptable bond of a blade body end inner wall with an end plate, it has been a practice to remove such detrimental surface and subsurface oxides from the hollow blade body by first vapor blasting then Ni plating the complex shaped hollow body end, without further treatment of the Ni plate. The end plate was prepared by acid cleaning. However, Ni plating of a complex shape such as a hollow air cooled airfoil has been relatively unpredictable and expensive. In addition, vapor blasting at the open blade tip of air cooled blades risks leaving entrapped vapor blasting media within the complex internal cooling passages of the blade, and can lead to decreased blade life during high temperature operation. Also, acid cleaning alone of Ni base superalloy blade tip caps or end plates does not provide an ideal surface for good brazing alloy flow: it is difficult to remove effectively all surface and near surface oxides from tip caps or end plates without exceeding design limits for intergranular attack of the plate substrate. As a consequence, prior brazed joints have been observed to include as much as 80-90 volume % oxides.